This type of underwater cutting granulating device is generally provided with a die plate having a large number of die holes, a water chamber formed so as to surround the cut surface of the die plate, a knife holder housed rotatably within this water chamber, a diving means for rotating the knife holder in a fixed direction, and a plurality of knives mounted on the knife holder.
A feed water supply system for circulating cooling water is connected to the water chamber. A resin feeder such as gear pump is connected to the die, so that a fused resin from a resin kneading machine is pressurized and supplied into the water chamber.
Accordingly, the fused resin extruded from the die into the water chamber, the skin of which is cooled at the same time the fused resin is pushed into the water chamber, is finely cut by the knives, cooled and hardened within the water chamber, and granulated into pellets.
Most of knives conventionally used for such an underwater cutting granulating device have, for example, a substantially triangular section as shown by the virtual line in FIG. 1 or a substantially trapezoidal section as shown in FIG. 16, wherein the rotating directional rear side (rear surface) is cut off vertically to the cut surface of the die.
When the rotating directional rear side (rear surface) of the knife is vertical to the cut surface in this way, the high-speed rotation of the knife increases the proportion of causing a turbulence in the rear thereof, which develops into a cavitation. Thus, the rotating resistance of the knife is increased by the turbulence itself or the entrainment of bubbles by the cavitation, and most of the driving force of the knife is consumed for this resistance, which contributed to a reduction in driving efficiency.
The large rotating resistance of the knife is linked to the higher flow velocity of a rotating directional water flow (different from the circulating flow of water by the feed water supply system) running along the cut surface of the die according to the rotation of the knife.
Therefore, when such a high M (melt flow rate) material with low viscosity that M exceeds 10 was used as the fused resin, the high-speed water flow trailing the knife caused a defect such as curve, deformation, crushing, hairline or the like (particularly remarkable with MR&gt;30) in the pellet shape after cutting.
Such a pellet having a shape defect is not only deteriorated in commercial value by the visual failure, but also caught in a hopper part in extrusion or extrusion molding, which often leads to a reduction in yield or reduction in operability of the underwater cutting granulating device.
In consideration of the circumstances as described above, the present invention aims at preventing the occurrence of a shape defect even in a resin material with high MFR by reducing the rotating resistance of knife to water to suppress the turbulence and cavitation and minimizing the generation of a turbulent part trailing the knife as much as possible.